The objective of this study is to define the phenotype and genetic basis of previously undescribed types of familial diabetes insipidus (FDI). All consenting members of kindreds in which novel types of FDI appear to be segregating will be admitted to the CRC for a series of tests to determine the presence, absence, cause, natural history, clinical concomitants and mode of interitance of the FDI. These evaluation will include a thorough history and physical examination as well as standard clinical measurements of basal fluid intake and urine output, posterior and anterior pituitary function, postural hemodynamics, the activity of the sympathetic and renin systems, MRI of the pituitary and hypothalamus and echocardiograms of the mitral and aortic valves. In addition, the genetic mutation responsible for the FDI will be mapped by testing for linkage of the phenotype to other diseases of known genetic locus and/or polymorphic VTRS (variable tandem repeat sequence) markers known to be located at close intervals throughout the human genome. This information will serve to improve the accuracy of phenotyping and clarify the pathogenesis of this and other types of FDI. It may also prove useful in understanding the genetic basis of other inherited disabilities of central nervous function.